Field of the Invention
The present invention relates generally to the fields of biomedical engineering, biochemistry, medical treatment, and surgical procedures. More specifically, the present invention provides methods, devices, and compositions for inducing changes in tissues, biomolecules, including bioactive molecules. These changes are notably useful for inducing alterations in tissues, most notably in skin, for cosmetic purposes.
Description of the Related Art
Heating of tissue is a fundamental physical event in many different medical procedures. Depending on the time-temperature history of the tissue, a cascade of physical, chemical, and biological events occurs when tissue is heated. These events can lead to a beneficial or deleterious response. One example of a beneficial response is the reduction or elimination of the appearance of skin wrinkles as a result of heat induced tissue contraction and skin thickening as a result of neocollagen formation following heat stimulation in tissues.
Skin wrinkles are often the consequence of advancing age and sun exposure. With increasing age and excessive sun exposure, skin quality deteriorates. This is due, in part, to changes in hydration and epidermal thickness, and on a molecular scale, to a decrease in the amount of collagen in the dermis. Further, subcutaneous fat accumulates or atrophies leading to furrowing of the skin, which produces wrinkles. In today's society, the appearance of skin wrinkles is often viewed negatively and so there is a desire in the community for a means to safely reduce or eliminate wrinkles.
For many years, wrinkles have been treated with chemical peels or mechanical dermabrasion, cosmetic medical procedures in which the surface of the epidermis of the skin, i.e., the stratum corneum, is removed chemically or by abrasion, such as sanding, respectively. In the late 1980s, laser ablation procedures for skin resurfacing were developed and approved.
Some of the first laser ablation procedures involved CO2 lasers, which ablated some or all of the outermost layer of the epidermis, the stratum corneum. The CO2 lasers could often generate enough heat in the dermis to cause a tissue contraction. This and subsequent repair of the epidermis and stratum corneum led to visible effects including wrinkle reduction and smoothing of the skin.
Nonetheless, inadvertent and lasting damage from burns to the epidermis was often evident, hypo- or hyper-pigmentation was fairly common, and patients receiving these treatments were required to stay indoors for weeks in order to avoid damaging ultraviolet rays from sunlight in their now unprotected dermal layers of skin. These lasers gave way to various lasers that operated at different wavelengths with the goal of reducing the negative effects. Today, laser procedures that are non-ablative and less damaging to surrounding tissues have replaced most of these original laser procedures. These lasers are much safer and produce much less damage to surrounding tissues, however much of the beneficial effects have also been lost, particularly with regard to skin tightening.
In the mid to late 1990s, another cosmetic technique for skin was developed that involves a non-ablative thermal alteration to skin. This procedure was based on concepts drawn from radiofrequency electrosurgical devices where electrical current is introduced into the patient via an electrode in electrical contact with tissue and exits through a ground electrode in contact elsewhere on the patient. These devices are referred as capacitive coupled devices whereby current flows between electrodes, and on the way, fields build up where tissues of relatively greater resistance are encountered.
In the case of skin, the stratum corneum and epidermis are only weakly conductive, so fields and heat build up there. Current flows through the conductive dermis, and again encounters resistance at the adiposal level, again resulting in heat generation. In skin, the treatment technique is referred to as radiofrequency (RF) skin rejuvenation (1).
In the skin, beneficial radiofrequency rejuvenation can result in tissue contraction as heat flows from the areas of field concentration, such as the adipose layer and epidermis, and into the dermis. Furthermore, a beneficial wound response to the heat in the dermis can lead to production of new collagen, and ultimately the skin may thicken. When treating the skin with RF devices, it is necessary to provide a conductive coupling gel between the skin and electrodes to allow for current flow.
Capacitive-coupled devices may result in negative, and sometimes severe consequences with regard to damage to tissues where the electric fields concentrate. Current generally follows the path of least resistance and thus it is not always predictable or controllable where its effects will occur. Any current that flows through the body is potentially hazardous. As the electric fields concentrate at non-conductive interfaces, electrical burns and heat damage may become evident at these interfaces. Burns are common at the electrodes in electrosurgical devices, and similarly, RF rejuvenation devices may also produce burns. In skin, capacitively-coupled radiofrequency heating exhibits preferential power absorption in the epidermis and in lower-conductivity subcutaneous fat. In other words, capacitive-coupled devices preferentially heat tissues with higher specific resistance (2-3). As a result, these tissues are at risk for damage.
To counter the effects of deleterious heating at the skin surface, capacitive-coupled skin rejuvenation devices (4), and lasers (5-6), often use some mechanism to cool the surface of the skin, thereby avoiding most of the damage to the outer epidermis and stratum corneum. Nonetheless, the risk of heating adipose tissue below the dermis is everpresent with RF devices and, anecdotally, patients have complained of long-term subcutaneous fat atrophy following treatment with these devices, with some of these requiring grafting. Efforts to reduce such detrimental effects require reduction of power output and have likely reduced efficacy of these devices.
More recently, additional devices for skin rejuvenation have been developed that employ ultrasound In an attempt to provide specific and localized treatment to the dermis. The devices focus the ultrasound within the dermis, or just below to achieve specific heating. Though specificity is improved, cavitation can result in pain and tissue damage. Burning and necrosis of the epidermis and stratum corneum during laser and RF cosmetic skin treatments is of major concern. Thus, various methods of skin cooling are often employed, including the spraying of cryogen on the skin surface or on an applicator, or applying cold air, water or ice to the skin.
In contrast to the aforementioned tissue heating devices and technology, magnetic induction applicators, such as those used in magnetic induction diathermy devices primarily induce (eddy) currents to flow along pathways governed by electric conductivities, hence depositing more power in tissues of higher conductivity (2). Inductively coupled diathermy units use induced eddy currents to heat tissue, especially tissue, such as muscle, with high water content (7), but only weakly affect tissues with high fat content (8). Nonetheless, diathermy devices are used for deep heating of tissue structures, and their effects on thin tissue layers such as the dermis have yet to be described.
Thus, there is a recognized, continuing need for improved methods and devices for specific heating of the thin dermal layer of skin with a high degree of specificity, efficacy and safety. Moreover, there is a recognized need for improvements in the use of magnetic induction methods and devices to heat tissue near or at the skin surface, and particularly, for specific dermal heating to achieve a cosmetic result.
The prior art is deficient in methods and devices for highly efficient and safe non-invasive heating of the skin, with high specificity for the dermis, while protecting collateral tissue structures. The present invention fulfills this longstanding need and desire in the art.